use it to clean the membrane?

I think there may be some lactose in the permeate:)

Use spinning hydrophilic nanofiltration membrane system to minimize the fouling.

Dr. Lin (membrane separation system technologist)

if it is very rich in ash then you have a lot of minerals in the permeate. Try using reverse osmosis to concentrate your permeate. You should try to define which minerals are present in the permeate before hand.

See this article http://dx.doi.org/10.1016/j.memsci.2004.06.029